This invention relates to brush wear indicators of the type used to sense and indicate a predetermined degree of brush wear in dynamoelectric machines having either commutators or slip rings on which brushes ride during normal operation of the machines.
It has long been recognized by those using dynamoelectric machines that it is often desirable to provide means for indicating a predetermined degree of brush wear in the machine so that those operating them will be alerted to replace the brushes in a timely manner and thereby avoid undesirably timed shutdown of the machinery. In response to that need a variety of different brush wear indicating mechanisms have been developed. A relatively early example of one such mechanism is shown in U.S. Pat. No. 2,691,114--Lykins which issued in Oct. 5, 1954. In that patent there is disclosed a torsion-spring-biased relatively rigid brush follower arm that is effective to move a switch contact arm against a fixed switch contact responsive to a predetermined degree of brush wear occurring. Closure of the switch contacts is effective to energize a light or other suitable signal means to alert an operator to the occurrence of a given amount of brush wear.
Another type of prior art brush wear mechanism is shown in U.S. Pat. No. 3,523,288--Thompson, which issued on Aug. 7, 1970. That patent discloses a system in which a spring-biased pin is mounted to slide along one side of a brush as the brush wears, then to drop into a recess at a preselected point near the outer end of the brush thereby causing a signal circuit switch to close with a snap action and alert an operator to the predetermined degree of brush wear. A quite similar brush wear indicating mechanism is shown in U.S. Pat. No. 4,121,207--Jones, which issued on Oct. 17, 1978 and discloses the use of an enclosed microswitch in combination with a switch actuating arm having a roller at its outer end that rolls along one side of a brush as it wears until the roller finally drops off the outer end of the brush and closes the switch with a snap action.
Finally, U.S. Pat. No. 4,172,988--Lowther, which issued Oct. 30, 1979 discloses a brush wear indicator assembly in which an electrical contact mounted on the outer end of a brush is moved into engagement with a fixed contact mounted on an insulating member adjacent the path of travel of the brush-mounted contact. As the brush is moved downward by a self-winding spring, responsive to the brush being worn away by a slip ring or commutator on which it rides, the brush-mounted contact eventually engages the fixed contact thus completing a circuit to an indicating signal that alerts the operator of the predetermined degree of brush wear.
All of the foregoing types of prior art brush wear indicating mechanisms, and other known prior art brush wear indicators, have certain disadvantages. Such brush wear indicator mechanisms are often relatively expensive to manufacture because they require the assembly of a number of additional parts that normally are not required in brush mounting and biasing structures. For example, the brush wear indicators shown in the Lykins and Lowther patents discussed above both require the provision of an extra movable switch contact that is actuated either by a rigid switch arm, as in the Lykins disclosure, or by being mounted directly on a brush, as is the Lowther disclosure. Alternatively, such prior art mechanisms are often relatively expensive because their implementation requires special machining and assembly procedures. For example, the Jones patent provides a special aperature in the wall of a brushholder and utilizes a separately mounted microswitch having a roller that operates through the aperture in the brushholder to engage the side of a brush in the holder.
A primary object of the present invention is to overcome the disadvantages inherent in prior art brush wear indicator assemblies and to provide a reliable and relatively inexpensive brush wear indicator.
A further object of the invention is to provide a brush wear indicator that utilizes a self-winding brush-biasing spring to actuate a signal means responsive to the occurrence of a predetermined degree of brush wear so that the spring performs a dual function of biasing the brush and sensing the predetermined degree of brush wear.
Still another object of the invention is to provide a brush wear indicator that utilizes a brush biasing spring to actuate a wear-signalling circuit responsive to the spring coiling on itself to actuate a switch mechanism.
Additional objects and advantages of the invention will become apparent to those skilled in the art from the description of it that follows considered in combination with the accompanying drawings.